Method and apparatus for impregnation of porous bodies for protection against oxidation

ABSTRACT

An open-pored body ( 11 ), in particular a carbon component of an aluminum production cell, which is to be exposed to oxidizing conditions or chemical attack at high temperatures is treated to protect the body against oxidation or corrosion at high temperature by impregnating the surface of the body at about ambient temperature with a hot non-saturated liquid ( 10 ). This liquid contains a treating agent at a temperature above the temperature of the body. The concentration of treating agent in the hot liquid is such that when the liquid is cooled, before it reaches the temperature of the body, the liquid saturates and treating agent precipitates. A pressure differential is applied to cause the liquid to impregnate into the surface pores of the body ( 11 ) and precipitate a layer of the treating agent from the liquid inside the body by cooling as it impregnates the pores of the body. The treating solution preferably comprises soluble boron compounds and, when the body is exposed to high temperature oxidizing conditions, the precipitated layer of the treating agent inside the body forms a protective, vitreous layer in the surface pores which protects the body against oxidation.

FIELD OF THE INVENTION

The invention relates generally to a method of treating open-poredporous bodies which are to be exposed to oxidising conditions at hightemperatures so as to protect the bodies against oxidation or otherchemical attack in said high temperature oxidising conditions, inparticular the treatment of prebaked carbon components of aluminiumproduction cells, such as anode blocks or cathode blocks or cell sidewalls.

The invention also relates to an apparatus for carrying out this methodand use of the apparatus for applying a treating liquid to a prebakedcarbon component of an aluminium production cell.

BACKGROUND OF THE INVENTION

The treatment of prebaked carbon components of aluminium productioncells, such as anode blocks, cathode blocks or cell sidewalls to improvetheir resistance to the conditions prevailing in the cell has alreadybeen proposed.

U.S. Pat. No. 5,486,278 (Manganiello et al) discloses treating aprebaked carbon-based anode of an electrolytic cell for the productionof aluminium, in particular by the electrolysis of alumina in a moltenfluoride electrolyte, over its sides and top to improve the resistancethereof to erosion and corrosion during operation of the cell by air andoxidising gases released at the anode, by immersing the anode in aboron-containing solution containing 5-60 weight % of H₃BO₃ or B₂O₃ inmethanol, ethylene glycol, glycerin or water with a surface-activeagent, e.g. at 80° to 120° C. After immersion, lasting up to an hour,the boron-containing solution is impregnated to a depth of usually about2-5 cm over the top and side surfaces of the anode to be protected,producing a concentration of boron in the impregnated surface from 100ppm to 0.35%. The same treatment can be applied to cell sidewalls.

It was found advantageous to carry out this treatment with a heatedsolution, but this involved heating of the anode, which consumed largequantities of energy. Attempts were therefore made to carry out theprocess at ambient temperature because no special heating equipmentwould be required. Low temperature application however required thecareful choice of solvents and surfactant agents in order to reduce thetreatment time as far as possible.

For prebaked anodes, only the top and top side surfaces need to beprotected, so it was suggested to dip the anode upside down into thesolution. But this is impractical when the anodes are fitted with rodsfor connection to a suspension device which also serves as a currentlead-in. Furthermore, it is inconvenient to treat the anodes first andthen fix the suspension rods.

To overcome this difficulty it would be possible to dip the anode in thetreating solution with the rodded top side up, and protect the bottompart of the anode by blocking its pores with a fugitive agent thatprevents impregnation with the boron-containing compound, and can beremoved afterwards. This however entails additional operations andcareful selection of the fugitive agent.

To speed up the process, it was suggested to assist the impregnation bythe application of a pressure differential, by pressure or vacuum.However, no practical way of doing this was disclosed.

U.S. Pat. No. 5,534,130 (Sekhar) describes the protection of the cellsidewalls of aluminium production cells by impregnating them with agentsbased on aluminium phosphate. Again, it would be desirable to perfectways of applying this method in an efficient manner.

SUMMARY OF THE INVENTION

It is an object of the invention to obviate the above-described problemsand shortcomings of the available methods and apparatus.

It is another object of the invention to provide a method of treating anopen-pored porous body which is to be exposed to oxidising conditions athigh temperatures so as to protect the body against oxidation orchemical attack in said high temperature condition, in particular carboncomponents of aluminium production cells such as anode blocks withoutnecessarily pre-heating the anode blocks or other bodies, while assuringan effective impregnation of the treating liquid into the pores of thetreated part, and enabling the treatment of large numbers of the bodiesin an efficient manner.

A particular object of the invention is to provide a method which canuse a hot treating liquid in a very efficient manner, withoutnecessarily heating the treated body substantially above ambienttemperatures.

In general terms, the invention provides a method of impregnating anopen-pored porous body with a hot treating liquid containing a dissolvedtreating agent to produce within the body a protective layer to adesired depth. This method comprises impregnating a hot non-saturatedtreating liquid into the surface of the porous body assisted by theapplication of a pressure differential. The applied treating liquid isat a temperature above that of the body and the concentration of thetreating agent in the applied liquid is such that cooling of the appliedliquid as it impregnates the pores of the body causes precipitation ofthe treating agent in the pores to the desired depth.

More specifically, the method according to the invention comprisesfirstly bringing the surface of the body into contact with a hot liquidcontaining a dissolved treating agent at a concentration belowsaturation and at a temperature well above the temperature of the body.The concentration of treating agent in the hot liquid is such that whenthe liquid is cooled down to the temperature of the body the liquidsaturates and treating agent precipitates.

A pressure differential is then applied to cause the hot non-saturatedtreating liquid contacting the body to impregnate into the surface poresof the body. As the treating liquid impregnates the pores of the body,it cools and deposits, within pores of the body underneath the surface,a layer of the treating agent precipitated from the impregnation liquid.Some treating agent may also cover the outside of the body's surface.

When, later, during use, the body is exposed to high temperaturesparticularly under oxidising conditions, this layer of the treatingagent precipitated inside the body underneath its surface forms aprotective layer in the surface pores which protects the body againstoxidation or other chemical attack. Advantageously, the treating agentcontains a soluble boron compound which forms an impervious viscousprotective layer which is self forming above about 400° C.

The treating liquid in contact with the body is usually at least 5° C.and in many cases at least 10° C. above the temperature at which thetreating liquid is saturated with the dissolved treating agent for itsgiven concentration.

Moreover, the treating liquid is at a temperature well above that of thebody to be treated, i.e. usually at least 20° C. above and conveniently40° C. or more above. For example the treating liquid is at atemperature in the range 60° to 120° C., whereas the body can be atambient temperature or just above, say from about 20° C. to 50° C. Theapplied treating liquid which is in contact with the body can bemaintained at a more-or-less constant temperature by heating it tocompensate for heat loss due to contact of the treating liquid with thebody.

The treating time can be reduced to a few minutes, which is a greatimprovement over the previous impregnation techniques.

There is no need to heat the body to be treated which makes the methodenergy efficient.

The method permits treatment of only a selected part of the body, inparticular by using a specially designed apparatus, described below.

Due to the rapidity of the impregnation process, the treated anode orother body only takes up a small quantity of heat, so the process isvery energy efficient.

The treated carbon bodies typically have an overall porosity in therange 20-24%, of which about 10-12% is open porosity. The penetrationdepth can be of the order of 0.5 to 3 cm. However, the treating agentmay also extend to and even cover the bodies' outer surface.

In one embodiment of the method, treating liquid is circulated by thefollowing arrangement. Hot treating liquid is supplied from a reservoirand, after treatment of the body, non-impregnated treating liquid isreturned back to the reservoir. The treating liquid in the reservoir isstirred and heated to maintain a desired temperature. Components of thetreating liquid (the dissolved treating agent; possible furthermaterials in suspension and water) can be added to the reservoir tocompensate for consumption of the treating liquid in the treatingprocess.

Another inventive aspect is a method of supplying the treating liquidwhich comprises providing a saturated solution of the treating agent ata given temperature, and deriving, from said saturated solution, anon-saturated solution of the treating agent. The non-saturated treatingsolution is then supplied as the treating liquid to treat the body.

For example, the non-saturated solution is obtained by increasing thetemperature of the solution. In this way, the non-saturated solutioncontains the treating agent at the same concentration as the saturatedsolution (which corresponds to the saturation concentration at saidgiven temperature) but is simply at a higher temperature.

The treating liquid preferably contains an oxidation retardant agent forimpregnating the part of the body to be treated, in particular at leastone soluble compound of boron and/or phosphorous for improving theresistance to oxidation of the carbon.

Preferred oxidation retardant agents for application to aluminiumproduction anodes are boron containing liquids based on B₂O₃, boricacid, tetraboric acid, salts of said acids or boron silicate. Theseboron containing agents such as B₂O₃ act as catalytic oxygen inhibitorswhen impregnated/dispersed in carbon. However, when they are denselyapplied they form a vitreous impervious layer in the surface pores attemperatures above about 400° C. Such a layer acts as a barrier toprotect the pores of a carbon body from oxidation. Such viscous layersact mainly by a barrier effect to inhibit oxidation. This is differentto the effect of deeply impregnated boron compounds whose effect isprincipally as an anti-catalyst.

Various silicon containing compounds, in particular boron silicate andsilica are also suitable for producing viscous protective layers at theoperating temperatures, especially in combination with boron compoundswhere an excess of the boron compound (in particular boric acid) willlead to the formation of borosilicate glass. Though the siliconcompounds alone are not effective against oxidation and provide noanti-catlytic effect, when combined with boron compounds the resultingglassy material provides improved protection.

Alternative treating liquids, specially suitable for protecting cellside walls, are liquids based on phosphates of aluminium for exampleselected from the group consisting of monoaluminium phosphate, aluminiumphosphate, aluminium polyphosphate, aluminium metaphosphate, andmixtures thereof.

The boron, phosphorous and/or silicon-containing treating liquids cancontain surfactants surfactant agents in particular tensio-activecationic agents. Anionic tensio-active agents can also be used. Suchagents should be devoid of components that would undesirably contaminatethe aluminium produced and components that promote oxidation of thecarbon. These surface-active agents may possibly be present togetherwith other solubility improving agents such as tartaric acid or citricacid, and the liquid may be heated to improve and to speed up theimpregnation of the anode.

When water is chosen as solvent, a surfactant such as those availableunder the tradenames NONIDET P 40 and SPAN 85, from Fluka, and GLUCOPON225, DEHYPON LS, QUAFIN LDM and QUAFIN CT, from Henkel, can be used inorder to achieve an acceptable low treatment time. However, this is notessential.

Optionally, treatment with the treating liquid may be combined with theapplication of a suspension containing particles which block the surfacepores of the body and form a surface coating, or the treatment may befollowed by the application of a suspension containing particles whichhelp reduce the pore size. Such a suspension may contain a colloidselected from colloidal alumina, silica, yttria, ceria, thoria,zirconia, magnesia, lithia, monoaluminium phosphate or cerium acetate,or particles of the same materials.

For certain applications, for example for treating the surface ofcathode blocks, this impregnation or top coating liquid may containparticulate refractory boride, such as TiB₂, and/or aluminium powder,chips or cuttings.

A very advantageous treating liquid further contains fine carbon powderwhich helps keep the pores blocked and helps prevent migration of theoxidation retardant agent (which becomes viscous at the operatingtemperatures) towards the bottom of the impregnated component.

Thus, the term “treating liquid” should be understood as includingnon-saturated solutions of the treating agent and “quasi-solutions”. Inaddition to the dissolved treating agent at below-saturationconcentration forming a true homogeneous solution, “quasi-solutions”further comprise components such as colloids and particles insuspension.

In all cases, when the treating solution is deposited in the pores ofthe treated body, the dissolved treating agent precipitates. Later, whenthe body is heated, removal of water will lead to a continuousprotective barrier layer substantially impervious to oxygen. For this,an adequate amount of the treating agent must be deposited in the pores,which can precisely be achieved with the method according to theinvention. Furthermore, to improve the action of closing of the pores bythe precipitated treating material, the treating solution advantageouslyfurther comprises the above-mentioned suspensions or powders.

For bodies which are exposed to uneven wear over different parts oftheir surface due to different oxidising conditions, for exampleprebaked carbon anodes or cell sidewalls of aluminium production cells,it is possible to apply different treatments to different areas in orderto optimize the protective effect against oxidation and in order tominimize the amount of oxidation retardant used. For example, it ispossible to impregnate practically the entire sides of anodes with adilute liquid of an oxidation retardant, as described in theaforementioned U.S. Pat. No. 5,486,278, followed by applying a surfacelayer in accordance with this invention only over the “top andshoulders” of the anode that will be recovered as the anode butt. Or the“top and shoulders” could be further protected by applying a top coatingincluding inert particles.

Where special problems of oxidation occur, it is also possible tofurther top coat or spray with a slurry for example of boric acid oranother protective material.

In a modified method, the surface of the body can first be impregnatedto a depth of several centimeters by impregnation with a “dilute”solution of the treating agent, followed by a treatment according to theinvention with a near-saturated solution which precipitates into thesurface pores.

The treated body may be a pre-baked anode of an aluminium productioncell, which anode is rodded, i.e. has a rod attached to its top face forconnection of the anode to a suspension device which also serves as acurrent lead-in. Using the method of the invention and the describedapparatus, rodded anodes can easily be treated with their rodded sideup, which greatly facilitates handling.

When preformed carbon anodes are put into service in aluminiumproduction cells, their bottom part immersed in the electrolyte is at atemperature usually about 960° C., whereas their top part exposed tooxidising gases is initially at a temperature of about 450-500° C., butwhich rises to 700° C. or more as the anode is consumed.

By protecting the top parts of the anodes in accordance with theinvention with a boron-containing liquid, at the operating temperaturethis liquid loses water and forms a highly viscous non-solid coating inthe surface pores of the top part of the anode to be protected. Thisviscous layer self-forms in the operating conditions and considerablyreduces wear of the top part of the anode by oxidation. As a result, theanode butts remaining at the end of the anodes useful life areconsiderably more intact than those of non-protected anodes, or anodesprotected by prior art methods.

The treated body may alternatively be part of a sidewall of an aluminiumproduction cell, or a cathode block of an aluminium production cell.

The method of the invention can advantageously be carried out in aspecially developed apparatus for applying a treating liquid to anopen-pored porous body. Such apparatus comprises a treating chamberhaving at least one sealing member which is arranged to be applied to abody to be treated which is placed in the treating chamber. The sealingmember or members is/are arranged so as to isolate an upper part of thetreating chamber around the part of the body to be treated from a lowerpart of the treating chamber around a bottom part of the body which isnot to be treated.

The apparatus also includes a reservoir containing a supply of hotnon-saturated treating solution of a treating agent at a temperatureabove the temperature of the body to be treated. The concentration oftreating agent in the hot solution is such that when the solution iscooled, before it reaches the temperature of the body, the solution willsaturate so that treating agent precipitates.

Means are provided for filling the upper part of the treating chamberwith a treating liquid to cover the part of the body to be treated.These means can include a pump for pumping treating liquid from a supplyreservoir.

Means are also provided for applying a pressure differential to intakean amount of the treating liquid into pores in the part of the body tobe treated, in particular by applying a vacuum to the lower part of thetreating chamber.

Lastly, means are provided for removing remaining treating liquid, whichhas not been absorbed by the body during the treatment, from thetreating chamber. These means can include another pump for pumpingresidual treating liquid back to the supply reservoir.

Further details of an advantageous apparatus are described in PCTapplication PCT/IB97/---- concurrently filed herewith (Ref. MOL0546-03).

In the method of the invention, a hot treatment liquid is to be used totreat bodies at ambient temperature. For this, the treating chamber canbe provided with means for heating the treating liquid in the upper partof the treating chamber to compensate for heat loss due to contact ofthe liquid with the body. Other arrangements to achieve a thermalbalance can be used, as appropriate.

An embodiment of the apparatus comprises a reservoir for treatingliquid, from which hot treating liquid is supplied to the top part ofthe treating chamber and to which remaining treating liquid, which hasnot been absorbed by the body during the treatment, is returned from theupper part of the treating chamber. This reservoir comprises means forheating and means for stirring the treating liquid therein. A meteringdevice can be provided for adding components of the treating liquid tothe reservoir to compensate for consumption of the treating liquid inthe treating process.

A preferred storage vessel or reservoir—which can be used for varioustreatments—comprises first and second compartments, the firstcompartment containing a reserve supply of the hot treating liquid incontact with a mass of the treating agent at a given temperature T₁. Inthe first compartment, the treating agent is dissolved at aconcentration which corresponds to the saturation concentration, attemperature T₁. The second compartment contains a supply ofnon-saturated treating liquid in which the treating agent is dissolvedat the same concentration but at a temperature T₂ above temperature T₁,or at the same temperature T₁ but a lower concentration. The vesselfurther comprises: means for maintaining the hot treating liquid in eachof the first and second compartments at the respective temperature; anoutlet conduit for supplying the non-saturated hot treating liquid fromthe second compartment of the vessel to treat a body or material; and aconduit for supplying hot treating liquid from the first compartment tothe second compartment to compensate for consumption/loss of thetreating liquid by treatment of the body or material.

The apparatus preferably comprises means for applying heat to treatingliquid in the upper part of the treating chamber to compensate forcooling of the liquid by contact with the body.

An advantage of the above-outlined method and apparatus is that it ispossible to treat large bodies such as prebaked anodes without a need topre-heat them. However, the bodies can be pre-heated if required, tojust above ambient temperature. In particular, it can be useful tolocally pre-heat the top part of the bodies to be treated, whilemaintaining a sufficient temperature differential with the treatingliquid.

Another advantage is that the method and apparatus allow sequentialtreatments to be carried out on the top part of the same body, over thesame area or over a different area, using the same treating liquid ordifferent treating liquids/slurries.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of an apparatus for carrying out themethod according to the invention.

FIG. 2 is schematic cross-sectional view of a preferred type ofreservoir for preparing and supplying the treating liquid.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically shows an apparatus for applying a treating liquid10 to a porous body 11 by the method of the invention. The apparatuscomprises a treating chamber 12 having an upper part 14 and a lower part15. The upper part 14 is open-topped and may be provided with aremovable cover or lid.

A sealing member 13 consisting of several sections of elastomericmaterial is arranged to surround a body 11 to be treated when the bodyis placed in the treating chamber 12 by inserting it into the open topby means of an automatic handling device (not shown). In the case ofsquare or rectangular bodies 11 such as prebaked carbon anodes ofaluminium production cells, four sections of elastomeric material can bearranged around the four sides, each section being associated with aseries of hydraulic, pneumatic or mechanically actuated cylinders, onesuch hydraulic cylinder 25 being shown in FIG. 1.

This sealing member 13, when it is tightened around the body 11,isolates a space in the upper part 14 of the treating chamber around thepart of the body 11 to be treated, from a lower part 15 of the treatingchamber around a bottom part of the body 11 which is not to be treated.

Isolation of the upper and lower parts 14, 15 of chamber 12 can beachieved by means of a flexible skirt associated with the sections ofthe sealing member 13, or by arranging the sections of the sealingmember 13 to fluid-tightly protrude from a groove or the like around thechamber wall.

A supply conduit 16 for treating liquid leads into the upper part 14 ofthe treating chamber. Conduit 16 leads from a reservoir 32 of treatingliquid 10 and has a supply pump 17 by means of which treating liquid 10can be supplied to the upper part 14 of the treating chamber so as tocover the part of the body 11 to be treated, up to a level determined bya sensor 22.

A Venturi or vacuum pump 18 is connected to the lower part 15 of chamber12 for evacuating the space around the underside of the body 11. Whenthe Venturi or vacuum pump 18 is switched on, air filling the pores ofbody 11 is evacuated, which causes an amount of the treating liquid 10in the upper part 14 of chamber 12 to be intaken into the part of thebody 11 to be treated. A pressure detector 19 is provided in the lowerpart 15 of chamber 12. This detector 19 is sensitive to the change inpressure which occurs when all of the pores of the part of body 11 beingtreated are filled.

An outlet conduit 20 is connected to the bottom of the upper part 14 ofthe chamber. This conduit 20 leads back to the reservoir 32 and has apump 21 for returning treating liquid remaining in the chamber at theend of treatment of a body 11 back to the reservoir 32.

A hydraulic system, comprising a hydraulic cylinder 25 controlled by ahydraulic pump 26, is provided for adjusting the sealing member 13 whichis connected to a piston of the hydraulic pump 26. When this piston ispulled into its cylinder 25, the member 13 is pulled out to allow aloose fit around a body 11, permitting insertion and removal of the body11 into or from the treating chamber 12. When the piston is pushed outof its cylinder 25, the sealing member 13 is tightened around the body11 to provide a sealing fit, by elastic deformation of the elastomericmaterial making up the sections of sealing member 13 when they areapplied against the body 11 under the pressure applied by the hydrauliccontrol.

Alternatively, it is possible to control the sealing member 13pneumatically, mechanically or electro-mechanically.

A position detector 30 is provided in the lower part 15 of the treatingchamber for detecting when a body 11 introduced into chamber 12 reachesa predetermined position. This position depends on the size of the body11 and corresponds to the level of the bottom of the body 11 when thetop of the body to be treated is at a level where it will be covered bytreating liquid 10 in the upper part 14 of the chamber. The detector 30is arranged to actuate the hydraulic pump 26 and hydraulic cylinder 25to bring the sealing member 13 to sealably engage with the body 11 whenthe body 11 has reached the given position.

The treating chamber 12 comprises a heater 31 for heating the treatingliquid 10 in the upper part 14 of chamber 12. The heater 31 can be anelectric heater or can operate by circulating hot air or another heatingfluid. This heater 31 can be adjusted to supply an amount of heat whichcompensates for heat loss due to contact of liquid 10 with body 11, i.e.depending on the size and temperature of the body 11 and its thermalcharacteristics, and the operating temperature of the treating liquid10. If required, the heater 31 can be replaced by means for maintaininga proper thermal balance.

The reservoir 32 supplies hot treating liquid 10 to the top part 14 ofthe treating chamber and, after the end of the treatment of a body 11,treating liquid remaining is returned to the reservoir 32 via conduit20. The reservoir 32 is externally insulated and is fitted with a heater33 for maintaining the treating liquid 10 at a selected temperaturecontrolled by a thermostat 37. A stirrer 34 constantly or intermittentlydriven by a motor 35 stirs the treating liquid 10 contained in reservoir32. In the embodiment shown in FIG. 1, at the top of reservoir 32 is ametering device 36 for adding components of the treating liquid 10 tothe reservoir 32 in an amount to compensate for consumption of thetreating liquid 10 in the treating process. The reservoir 32 alsoincludes a pressure sensor 38 for monitoring the level of liquid 10 anda sensor 39 measuring the density or the conductivity of liquid 10,serving to control the metering device 36.

The treating liquid 10 in reservoir 32 is maintained at a selectedtemperature, say from 60° C. to 120° C. and contains a treating agent ata concentration well below saturation. An advantageous reservoir formaintaining a supply of non-saturated treating liquid is described belowwith reference to FIG. 2.

A preferred apparatus comprises a control panel schematically indicatedat 40. This control panel includes controls for all routine adjustmentssuch as temperature of the liquid in reservoir 32 and in the top part 14of chamber 12, the liquid level to be controlled by sensor 22, etc. Thecontrol panel also includes an overall control arranged to sequentiallyperform the following operations:

First, the sealing member 13 is actuated (by the position detector 30and hydraulic control 26/25) to sealably engage with the body 11 whenthe body 11 to be treated has reached the given position.

Next, the pump 17 is switched on to fill the upper part 14 of thetreating chamber 12 with a quantity of hot treating liquid 10 from thereservoir 32, up to a level controlled by sensor 22.

Then, the lower part 15 of the treating chamber is evacuated byswitching on the Venturi or vacuum pump 18, which evacuates air from thepores of body 11 and intakes treating liquid 10 into the surface pores.

During the vacuum treatment, if required the heater 31 is switched on toheat the treating liquid 10 in the upper part 14 of the chamber 12 inorder to compensate for cooling of the liquid 10 by contact with body11, so that the treating liquid 10 in contact with the body 11 remainsat more-or-less constant temperature above the saturation temperature.In this way, the treating liquid in the circulation system is alwaysmaintained non-saturated, thereby avoiding unwanted deposits of thetreating agent in the circulation system.

When the vacuum is applied, hot treating liquid is intaken into thepores of body 11 which, for example, is at ambient temperature. As it isintaken into the pores, the treating liquid heats the surface part ofbody 11, whereas the temperature of the impregnated treating liquiddrops. As the pores fill with treating liquid, the deeper the liquidpenetrates the cooler it becomes until it deposits precipitated treatingagent firstly in some pores. The liquid is then diverted to fill thosepores which still remain open. When all pores are full and blocked bydeposited treating agent, the impregnation is sufficient and this issignaled by the pressure detector 19, which immediately or after a giventime automatically switches off the Venturi or vacuum pump 18 and thenactuates pump 21 to remove residual treating liquid 10 from the upperpart 14 of the treating chamber.

When all of the residual liquid 10 has been removed from the upper part14 of the treating chamber, the hydraulic pump 26 is actuated to releasethe sealing member 13 and allow removal of the treated body 11 from thetreating chamber 12 by an automatic handling device (not shown).

A typical body 11 to be treated is a prebaked carbon anode of analuminium production cell. Such bodies usually have a porosity of about20-24%, of which 10-12% is open porosity. A prebaked anode may weigh ofthe order of 1000 kg. Attached to its upper side of the anode is a steelrod for connection to a suspension device and which also serves as anelectrical connection.

The treatment of prebaked anodes by known impregnation processes isdifficult (because of the attached rod) and energy consuming (because ofthe need to heat the entire anode to a temperature at which theimpregnation process will be effective).

With the method according to the invention, the treatment of prebakedanodes is advantageous, because the anode can be treated with its roddedside up and only the part which needs to be treated (the upper sideshoulders and top) can be treated without preheating of the anode, in asimple treatment with the anode at ambient temperature and the treatingliquid at a convenient temperature, say from 60° to 120° C. in atreatment lasting only a few minutes.

Moreover, due to the rapidity of the impregnation process, the treatedanode or other body only takes up a small quantity of heat, so theprocess is very energy efficient.

In an alternative embodiment, the operating cycle is slightly modified,by removing used treating liquid from the bottom part 15 of the chamberafter releasing the sealing members 13. In this case, when the sealingmembers 13 are retracted, the used treating liquid flows down and iscollected in the lower part of the treating chamber, in a channel orsump at the bottom of an inclined surface down which the liquid flows.The outlet 20 then leads from this channel or sump for return of theused liquid to the reservoir. Particularly when boron-based orphosphorous-based treating liquids are used, a sloping floor ensurescontinuous draining of the used liquid, which avoids unwanted depositsof the oxidation retardant. This simplifies maintenance of the apparatusand reduces operating costs.

FIG. 2 shows the main components of a very advantageous design of astorage vessel or reservoir 52 for supplying the treating liquid 10 viaa supply conduit 16 and returning used treating liquid to the reservoir52 via a return conduit 20.

Reservoir 52 is separated by a horizontal inner divider wall 54 into anupper (or first) compartment 55 and a lower (or second) compartment 56.The upper compartment 55 contains saturated treating liquid 10″ at atemperature T₁, this saturated treating liquid 10″ being in contact witha mass 60 of undissolved treating agent on the divider wall 54. Thelower compartment 56 contains a supply of non-saturated treating liquid10 which is at the same concentration as that in compartment 55 but isat a temperature T₂ which is higher than T₁.

The reservoir 52 further comprises arrangements for maintaining the hottreating liquid in the compartments 55 and 56 at the respectivetemperatures T₁ and T₂. As shown for compartment 55, such an arrangementcomprises an outlet 61 and a return inlet 62 for circulating the hotliquid via a standard type of heat exchanger 63 which heats thecirculating liquid to the desired temperature T₁ (or T₂). A similararrangement (not shown) is provided for compartment 56. Theseheater/circulating arrangements also serve to stir the liquid incompartments 55 and 56. Additional stirrers can be included if desired.

Compartment 55 has an outlet 64 and compartment 56 has an inlet 65 viawhich hot treating liquid 10″ from compartment 55 at temperature T₁ canbe transferred into compartment 56 where the liquid is maintained at atemperature T₂ above T₁. Heating the liquid from temperature T₁ to T₂can be done between the outlet 64 and inlet 65, or in a separateheating/circulating device (like 61, 62, 63).

The reservoir 52 also has a window 66 in one face, through which thelevel of the undissolved treating material 60 can be monitored. As shownfor compartment 55, a thermometer 67 and a manometer 68 are provided formonitoring the temperatures T₁ (or T₂) and the pressure in therespective compartment 55 (or 56).

At the top of reservoir 52 is a hinged cover 70 which can be manuallyopened for tipping into the compartment 55 a fresh supply of material 60when needed, as can be visually ascertained by inspection via window 66.The material 60 can thus be supplied at convenient times by emptying it,for example from a sack. The cover 70 is fitted with a seal to preventthe escape of fumes.

At the bottom of reservoir 52 is an optional manhole-type opening 71that can be opened for manually removing debris etc. that may accumulatein the lower compartment 56. For this purpose, the lower compartment isfitted with a perforated dividing wall 76 for restraining the debris.

The reservoir further comprises a hot water inlet 72 and a hot wateroutlet 73 at the top of compartment 55. It thus possible to include,inside the main compartment 55 of the reservoir 52, an internal hotwater storage tank arranged so that, when needed, the stored hot watercan be used to flush the conduits 61, 62 and the heat exchanger 63 todissolve any deposits of the treating material deposited from thesaturated liquid.

Also, each of the upper and lower compartments 55, 56 has a ventingdevice for equalizing pressure therein. This consists of a vent tube 74,which connects the compartment 56 to the outside, having aspring-actuated closure flap 74′ for venting in case of excess pressurein the compartment 56. Likewise, compartment 55 has a vent tube 75closed by a spring-actuated closure flap 75′.

The described improved reservoir 52 can be used for supplying hottreating liquid for various processes; i.e. even without using a vacuum.For instance, it can be used to spray or otherwise apply a topcoating ofthe treating material onto a treated body. This reservoir 52 is veryadvantageous from several points of view.

By maintaining the treating liquid at a given temperature T₁ incompartment 55, the treating material therein is maintained dissolved,at a concentration which corresponds to the saturation concentration atthat temperature T₁. This is achieved without any complex control meansand without a need to meter in selected amounts of the treatingmaterial, simply by maintaining an excess of undissolved treatingmaterial 60, by adequate stirring, and by maintaining the temperature atthe desired value T₁. Dissolved at the same concentration as before, butis sufficiently below the saturation concentration that when the liquidis supplied for example to the upper part of chamber 14, the risk ofunwanted deposition of the treating material in the supply arrangementis reduced or eliminated, compared to when a liquid at or nearsaturation is used. Moreover, the concentration of the treating liquidis controlled in a very simple way. The reservoir 52 can be filledmanually at convenient intervals with fresh treating material 60. Thesimple arrangement with sealed cover 70 avoids the escape of undesirablefumes.

Instead of increasing the temperature in compartment 56, the same effectcould be achieved by maintaining the treating liquid at the temperatureT₁ and adding selected amounts of hot water at temperature T₁.

Instead of returning the used treating liquid into compartment 56 viathe conduit 70, it could be returned into compartment 55. For certainapplications of the reservoir 52, the supplied treating liquid need notbe returned to the reservoir.

When a solution of boric acid is used for the treatment of the upperparts of pre-baked carbon anodes of aluminium production cells, asaturated solution of boric acid can be contained in compartment 55 at atemperature T₁ in the range 50° C. to 110° C., for example. Thesub-saturated treating solution 10 can be obtained by heating thesolution in compartment 56 to a temperature say about 10° C. to 15° C.above T₁. When this sub-saturated solution is impregnated into thesurface of a carbon anode 11 at ambient temperature assisted by vacuum,and maintaining the external solution hot by means of the heater 31, aboron-containing layer precipitates in the pores underneath the surfaceof the carbon body 11. During use of the anode, the impregnated layervitrifies and forms a dense viscous protective layer considerablyreducing oxidation of the upper part of the anode which remains as ananode butt.

What is claimed is:
 1. A method of treating an open-pored porous body(11) which is to be exposed to oxidising conditions or other chemicalattack at high temperatures so as to protect the body against oxidationor chemical attack at said high temperatures, which method comprises: A)bringing into contact with the surface of the body (11) at a temperaturewhich is about or just above ambient temperature a hot treating liquid(10) containing a dissolved treating agent at a concentration belowsaturation concentration, the hot treating liquid being at a temperatureabove the temperature of the body and the concentration of treatingagent in the hot treating liquid being such that when the liquid iscooled down to the temperature of the body, the liquid saturates and thetreating agent precipitates; B) applying a pressure differential tocause the treating liquid to impregnate into the surface pores of thebody and deposit, within pores of the body (11) underneath the surface,a layer of the treating agent precipitated from the liquid inside thebody by cooling as the liquid impregnates the pores of the body, and C)controlling said bringing into contact of the surface of the body (11)with the treating liquid (10) and applying said pressure differential sothat the temperature of the body remains below the temperature of thetreating liquid during the entire duration of their contact; saidprecipitated layer of the treating agent inside the body forming, whenthe body is exposed to said high temperatures, a protective layer in thesurface pores which protects the body against oxidation or otherchemical attack.
 2. The method of claim 1, wherein said treatingsolution comprises a soluble boron compound which forms a viscousprotective layer at temperatures above 400° C.
 3. The method of claim 1,wherein application of the pressure differential fills said pores withtreating liquid, the application of the pressure differential beingcontinued at least until all of the pores of the part of the body beingtreated are filled.
 4. The method of claim 1, wherein the pressuredifferential is applied by applying a vacuum to a part of the body outof contact with the treating liquid.
 5. The method of claim 1, whereinthe treating liquid is at a temperature at least 5° C. above thetemperature at which the treating liquid is saturated with the treatingagent.
 6. The method of claim 5, wherein the treating liquid is at atemperature at least 10° C. above the temperature at which the treatingliquid is saturated with the treating agent.
 7. The method of claim 5,wherein the applied treating liquid is at a temperature at least 20° C.above the temperature of the body to be treated.
 8. The method of claim5, wherein the treating liquid is supplied at a temperature in the range60° C. to 120° C.
 9. The method of claim 8, wherein the treating liquidin contact with the body is maintained at said temperature in the range60° C. to 120° C. by heating it to compensate for heat loss due tocontact of the treating liquid with the body (11).
 10. The method ofclaim 7, wherein the treated body is at a temperature in the range 20°C. to 50° C.
 11. The method of claim 1, comprising supplying hottreating liquid (10) from a reservoir (32,52) to the body (11) to betreated; returning non-impregnated treating liquid after treatment backto the reservoir (32,52); and adding components of the treating liquidto the reservoir to compensate for consumption of the treating liquid inthe treating process.
 12. The method of claim 1 which comprisesproviding a saturated solution (10″) of the treating agent at a giventemperature (T₁), and deriving from said saturated solution a treatingliquid comprising a non-saturated solution (10) of the treating agent.13. The method of claim 12, wherein the treating liquid comprising saidnon-saturated solution (10) is derived from the saturated solution (10″)by raising its temperature, the non-saturated solution containing thetreating agent at the same concentration which corresponds to thesaturation concentration at said given temperature (T₁), but being at ahigher temperature (T₂).
 14. The method of claim 1, comprisingsequentially: a) applying hot treating liquid to the surface of the bodyto be treated; b) applying a vacuum to a part of the body not to betreated; c) supplying heat to treating liquid in contact with the bodyto compensate for heat loss due to contact of the liquid with the body;d) stopping the application of a vacuum; and e) removing non-impregnatedtreating liquid.
 15. The method of claim 11, wherein the treating liquidcontains at least one soluble compound of boron and/or phosphorous. 16.The method of claim 15, wherein the treating liquid is aboron-containing liquid based on B₂O₃, boric acid, tetraboric acid, orsalts of said acids.
 17. The method of claim 15, wherein the treatingliquid contains monoaluminium phosphate, aluminium phosphate, aluminiumpolyphosphate, aluminium metaphosphate or a mixture thereof.
 18. Themethod of claim 15, 16 or 17, wherein the treating liquid furthercontains a silicon compound such as silica or boron silicate.
 19. Themethod of claim 1, wherein the treating liquid further contains asuspension of particulate material which assists in blocking the poresof the body and/or in the formation of the protective layer.
 20. Themethod of claim 19, wherein said suspension contains at least one of: acolloid selected from colloidal alumina, silica, yttria, ceria, thoria,zirconia, magnesia, lithia, monoaluminium phosphate or cerium acetate; asuspension of alumina, silica, yttria, ceria, thoria, zirconia,magnesia, lithia, monoaluminium phosphate or cerium acetate; particulaterefractory boride; aluminium powder, chips or cuttings; carbon powder;and mixtures thereof.
 21. The method of claim 1, wherein said layer oftreating agent is applied selectively to at least one part of the body,and at least one further treatment is applied to at least one other partof the body.
 22. The method of claim 1, wherein at least part of saidlayer of treating agent is topcoated by the application of a protectivematerial.
 23. The method of claim 1, wherein the treated body is apre-baked carbon anode (11) of an aluminium production cell.
 24. Themethod of claim 23, wherein the anode (11) comprises a rod attached toits top face for connection of the anode to a suspension device and forelectrical supply, the top part of the anode being treated with therodded top face of the anode up.
 25. The method of claim 1, wherein thetreated body is part of a carbon sidewall of an aluminium productioncell.
 26. The method of claim 1, wherein the treated body is a carboncathode block of an aluminium production cell.
 27. An apparatus forapplying a treating liquid to an open-pored porous body at a temperaturewhich is about or just above ambient temperature, by the method of claim1, the apparatus comprising: A) a reservoir (32,52) containing a supplyof hot treating liquid (10) containing a dissolved treating agent at aconcentration below saturation concentration, the hot treating liquid(10) being at a temperature above the temperature of the body (11) andthe concentration of treating agent in the hot treating liquid beingsuch that when the liquid is cooled down to the temperature of the body,the liquid saturates and the treating agent precipitates; B) a treatingchamber (12) and at least one sealing member (13) arranged to be appliedagainst a body (11) to be treated placed in the treating chamber, so asto isolate an upper part (14) of the treating chamber around the part ofthe body to be treated from a lower part (15) of the treating chamberaround a bottom part of the body which is not to be treated; C) means(16,17) for supplying the hot non-saturated treating liquid from thereservoir into the upper part (14) of the treating chamber to contactthe part of the body (11) to be treated; D) means for bringing intocontact the surface of the body (11) at a temperature which is about orjust above ambient temperature with the hot treating liquid (10); E)means (18) for applying a pressure differential to the bottom part ofthe body not to be treated, so as to intake an amount of the treatingliquid (10) into pores in the part of the body (11) to be treated andthereby deposit, within pores of the body underneath the surface, alayer of the treating agent inside the body by cooling as it impregnatesthe pores of the body; and F) means (20,21) for removing unused treatingliquid, said means D) and said means E) being arranged to control saidbringing into contact of the surface of the body (11) with the treatingliquid (10) and to apply said pressure differential so that thetemperature of the body remains below the temperature of the treatingliquid during the entire duration of their contact.
 28. The apparatus ofclaim 27, wherein the treating chamber (12) comprises means (31) forheating treating liquid in the upper part (14) of the treating chamberto compensate for heat loss due to contact of the liquid with the body(11).
 29. The apparatus of claim 27, wherein the reservoir (32)comprises means (36) for adding components of the treating liquid to thereservoir (32) to compensate for consumption of the treating liquid inthe treating process.
 30. The apparatus of claim 29, wherein thereservoir (52) comprises first and second compartments (55, 56), thefirst compartment (55) containing a reserve supply (10″) of the hottreating liquid in contact with a mass (60) of the treating agent at agiven temperature (T₁) at which the treating agent is dissolved at aconcentration which corresponds to the saturation concentration at thattemperature; the second compartment (56) containing a supply ofnon-saturated treating liquid (10) in which the treating agent isdissolved at the same concentration but at a temperature (T₂) above saidselected temperature (T₁) or at the same temperature but a lowerconcentration, the reservoir (52) further comprising: (a) means(61,62,63) for maintaining the hot treating liquid in each of the firstand second compartments (55,56) at the respective temperature; (b) anoutlet conduit (16) for supplying the non-saturated hot treating liquid(10) from the second compartment (56) of the vessel to treat a body; and(c) a conduit (64,65) for supplying hot treating liquid (10″) from thefirst compartment (55) to the second compartment (56) to compensate forconsumption/loss of the treating liquid by treatment of the body. 31.The apparatus of claim 27, wherein the treating liquid contains at leastone soluble compound of boron and/or phosphorous.
 32. The apparatus ofclaim 31, wherein the treating liquid is a boron-containing liquid basedon B₂O₃, boric acid, tetraboric acid, salts of said acids, or boronsilicate.
 33. The apparatus of claim 31, wherein the treating liquidcontains monoaluminium phosphate, aluminium phosphate, aluminiumpolyphosphate, aluminium metaphosphate or a mixture thereof.
 34. Theapparatus according to claim 27, wherein the treating liquid furthercontains at least one silicon compound such as silica or boron silicate.35. The apparaus according to claim 27, wherein the treating liquidcontains a suspension of particulate material which assists in blockingthe pores of the body.
 36. The method of claim 35, wherein saidsuspension contains at least one of: a colloid selected from colloidalalumina, silica, yttria, ceria, thoria, zirconia, magnesia, lithia,monoaluminium phosphate or cerium acetate; a suspension of particulatealumina, silica, yttria, ceria, thoria, zirconia, magnesia, lithia,monoaluminium phosphate or cerium acetate; particulate refractoryboride; aluminium powder, chips or cuttings; carbon powder; and mixturesthereof.
 37. The apparatus of claim 27, wherein the lower part (15) ofthe treating chamber (12) has a sloping surface leading to a sump orchannel for collecting remaining treating liquid.